Billionaire investor Warren Buffett told CNBC on Monday that if someone invested $10,000 in an index fund back in 1942, it would be worth $51 million today.

“The best single thing you’ve could have done on March 11, 1942 — when I bought my first stock — was buy an index fund,” Buffett told CNBC’s “Squawk Box.”

Since that time, Buffett said that Americans have seen 14 U.S. presidents, a world war, 9/11 and the Cuban Missile Crisis. “The only thing you had to believe in [1942] is that America would win the war and that America would progress as it has ever since 1776,” he said. “The headlines were terrible every day.”

Buffett, who was 11-years-old in 1942, said that people can be enticed into the stock market at the wrong time, adding all investors need to do is “buy a cross-section of America.”

“Most people get excited about a stock or the market after it’s gone up,” Buffett, now 87, said. “I mean their neighbor made a lot of money and they know they’re smarter than their neighbor. … It’s very frustrating.”

“It’s best to buy over time,” he added.

Buffett appeared on CNBC from Omaha, Nebraska where Berkshire Hathaway held a weekend of events around Saturday’s annual meeting.

Buffett, chairman and CEO of Berkshire, and his longtime investing partner and vice chairman, Charlie Munger, spoke to tens of thousands of attendees on a wide range of topics from their massive stake in Apple to missing out Alphabet‘s Google and Amazon to bashing bitcoin as “rat poison.”

On Friday, Buffett revealed that Berkshire Hathaway bought 75 million shares of the tech giant during the first quarter. That adds to the 165.3 million shares Berkshire already owned at the end of 2017.

Berkshire first made an investment in Apple in 2016 after a person at the company bought about 10 million shares. Buffett then looked at the stock and purchased considerably more, the billionaire recalled in August to CNBC.

— Buffett joins “Squawk Box” for three hours, 6 a.m. ET to 9 a.m. ET, with special guests Munger and Microsoft co-founder and philanthropist Bill Gates.

With Berkshire’s 2018 annual meeting in the books, users can revisit the highlights in CNBC’s Warren Buffett Archive, which houses searchable video from 25 full annual meetings, going back to 1994, synchronized to 2600 pages of transcripts. The Warren Buffett Archive also includes 500 shorter-form videos arranged by topic, CNBC interviews, a Buffett Timeline, and a Berkshire Portfolio Tracker.